US police on Saturday charged a woman biology professor with murder after three staff members were killed and three injured in a shooting spree at an Alabama university.

Amy Bishop Anderson, 45, a mother of four, was charged with capital murder and could face other charges including aggravated assault, district attorney Rob Broussard told a press conference in the southern town of Huntsville.

Police said she used a 9 mm weapon, armed with 16 bullets, which was later found in the womenââ‚¬â„¢s bathroom.

A stunned Bishop, dressed in jeans and a pink sweater, was seen being driven away from the University of Alabama in Huntsville by police after the incident, shaking her head in disbelief.

ââ‚¬Å“It didnââ‚¬â„¢t happen. Thereââ‚¬â„¢s no way. Theyââ‚¬â„¢re still alive,ââ‚¬ she murmured to local television station WHNT-TV as she climbed into the vehicle.

The shooting allegedly happened after Bishop, who had worked at the university since 2003, discovered several months ago she had been denied tenure, which would have secured her job in the biology faculty.

Witnesses told local media screaming had broken out as the biology faculty met Friday in the math and science building, the Shelby Center.

The three slain faculty members were identified as Gopi Polia, the chair of the biology department; Maria Ragland Davis, a professor of biotechnology; and Adriel Johnson, a professor of physiology.

Two of the injured staff members were said to still be in a critical condition on Saturday, and the other was stable.

University president David Williams told AFP Saturday of his shock, saying his first reaction had been: ââ‚¬Å“This canââ‚¬â„¢t be happening. Itââ‚¬â„¢s incomprehensible.ââ‚¬

The university, which has about 300 staff, has a ââ‚¬Å“no-gunââ‚¬ policy on campus, he said. ââ‚¬Å“We do not have metal detectors on our campus. This is a very safe community and it was a safe campus.ââ‚¬

An e-mail alert sent to students Friday read: ââ‚¬Å“There has been a shooting on campus. The shooter has been apprehended. Everyone is encouraged to go home, classes are cancelled tonight... Counselors are available.ââ‚¬

Williams confirmed about a dozen people had attended the biology faculty meeting, and said Bishop, who he did not know well, had been informed several months ago that she would not be getting tenure.

ââ‚¬Å“A typical tenure process is seven years since hiring for an assistant professor... It is not unusual for a professor to be employed for six years before a final decision is made,ââ‚¬ he said.

He said his first focus was helping the students deal with their grief and to help rebuild the college and the university.

The students are strong and ââ‚¬Å“will help us get through this. Weââ‚¬â„¢ve already come together and this will make us a stronger campus.ââ‚¬

The Huntsville Times said Bishop, a Harvard-educated geneticist and her husband, Jim Anderson, are credited with inventing a mobile cell incubation system, which could replace the old-fashioned petri dish.

The incident was just the latest in a series of school shootings to rock the United States ââ‚¬" most of which have been carried out by students ââ‚¬" amid the nationââ‚¬â„¢s ever-prevalent debate about gun control.

The shooting comes more than two years after the southern state of Virginia was stunned by the April 2007 massacre of 32 people at the Virginia Tech university by a student gunman, Seung-Hui Cho, who turned his gun on himself.

In 1999, two teenagers went on the rampage at Columbine school, Colorado, gunning down 13 people before also killing themselves.

In the first six weeks of this year alone several shootings have already been reported around the country.

Last month, eight people were killed in Virginia by a lone gunman. And in early January a disgruntled employee at a Missouri plant of Swiss power company ABB went on a rampage shooting dead three people and wounding five others, before killing himself.